Microelectronic devices generally have a die (i.e., a chip) that includes integrated circuitry having a high density of very small components. In a typical process, a large number of dies are manufactured on a single wafer using many different processes that may be repeated at various stages (e.g., implanting, doping, photolithography, chemical vapor deposition, plasma vapor deposition, plating, planarizing, etching, etc.). The dies typically include an array of very small bond-pads electrically coupled to the integrated circuitry. The bond-pads are the external electrical contacts on the die through which the supply voltage, signals, etc., are transmitted to and from the integrated circuitry. The dies are then separated from one another (i.e., singulated) by dicing the wafer and backgrinding the individual dies. After the dies have been singulated, they are typically “packaged” to couple the bond-pads to a larger array of electrical terminals that can be more easily coupled to the various power supply lines, signal lines, and ground lines.
Electronic products require packaged microelectronic devices to have an extremely high density of components in very limited space. For example, the space available for memory devices, processors, displays, and other microelectronic components is quite limited in cellphones, PDAs, portable computers, storage devices, media players and many other products. As such, there is a strong drive to reduce the surface area or “footprint” of a microelectronic device on a printed circuit board, lead frame, or other type of substrate. Reducing the size of a microelectronic device is difficult because high performance microelectronic dies generally have more bond-pads that result in larger ball-grid arrays and thus larger footprints.
“Flip-chip” packages are attractive for such high performance, small microelectronic devices. These devices are referred to as “flip-chips” because they are typically manufactured on a wafer and have an active side with bond-pads that initially face upward. After completing the die, it is singulated and inverted or “flipped” such that the active side bearing the bond-pads faces downward for attachment to a substrate. The bond-pads are usually coupled to terminals, such as conductive “bumps,” that electrically and mechanically connect the die to the substrate. The bumps on the flip-chip are generally formed from solder, conductive polymers, or other materials. When the bumps are made from solder, they are reflowed to form discrete solder joints between the flip-chip component and the substrate. This leaves a small gap between the die and the substrate. To enhance the integrity of the joint between the die and the substrate, an underfill material is generally introduced into the gap. The underfill material bears some of the stress placed on the components and protects the components from moisture, chemicals, and other contaminants. The underfill material can include filler particles to increase the rigidity of the material and modify the coefficient of thermal expansion of the material.
Most flip-chip devices use a lead-tin solder that requires flux to remove oxide during assembly. Although lead-tin solders provide high yields and reliable connections, soldering generally involves potentially hazardous materials and presents other challenges. First, it is generally costly and inefficient to handle hazardous materials. Second, the temperatures of reflow processes may be above the upper limits for some of the materials used in the packages. Third, solder interconnects are relatively large compared to gold-to-gold interconnects. Many solder-based flip-chip packages accordingly require a redistribution layer on the die that redistributes the very fine pitch of the bond-pads to an array having a larger pitch to accommodate the solder interconnects.
Gold-to-gold interconnects are one alternative to solder interconnects. Gold-to-gold interconnects generally have gold stud bumps placed on the die bond-pads through a modification of the “ball bonding” process used in conventional wire-bonding. In ball bonding, the tip of the gold bond wire is melted to form a sphere, and the bonding tool presses this sphere against a bond-pad while applying mechanical force, heat and ultrasonic energy to create a metallic connection. The gold is broken just above the ball to form a gold ball or “gold stud bump” on the bond-pad. After placing the gold stud bumps on a chip, they may be flattened by mechanical pressure to provide a flat-top surface and uniform bump height (i.e., co-planarity). Gold stud bumps are relatively easy to form with conventional wire-bonding equipment, and they do not use hazardous materials that require expensive and sophisticated handling processes. Gold stud bumps can also be quite small, and are thus very useful for fine pitch arrays with a large number of very small bond-pads.
Gold stud bumps, however, have only been used in limited applications because it is challenging to use them in many types of packaged devices. For example, because gold stud bumps are quite small, they must have good co-planarity and there must be good parallelism between the die and substrate to obtain good diffusion bonding. Misalignment between the die and the substrate, or non-uniform bump heights, may cause openings in the interconnects. As a result, gold stud bumps are not used in high density arrays on laminate substrates formed from organic dielectric materials. Additionally, large arrays require more ultrasonic power and greater down forces to attach the gold stud bumps to the bond-pads, and this can damage the dies. Therefore, it would be desirable to develop a packaged device and a method for packaging devices in which gold stud bumps can be formed in large arrays on organic substrates or other types of substrates to enable gold interconnects to be used in a wider range of applications.